Diapausing adult mosquitoes may provide a key overwintering refuge for vector-borne disease pathogens and, in so doing, play a significant role in the transmission of encephalomyelitis and malaria. Despite the potential epidemiological importance of mosquito species that undergo adult reproductive diapause, their physiology and behavior remain largely uninvestigated. We propose to examine the hormonal and sensory aspects of adult diapause in two species of mosquito (Culex tarsalis and Anopheles freeborni) within the context of two behavioral repertoires: host-seeking and sexual receptivity. The methods employed will include 1) assessment of previtellogenic ovarian follicle development, 2) measurement of host-seeking behavior and antennal lactic acid receptor sensitivity, 3) allatectomy and corpora allata implantation, and 4) juvenile hormone homolog and analog replacement. First, the course of diapause entry and maintenance will be described in terms of physiology, behavior, and receptor sensitivity. The relationship between receptor sensitivity and behavior will be evaluated. The lability of the diapause state in response to transient temperature increases will be exmained. The necessity of sexual receptivity to diapause entry will be determined. Finally, the hormonal basis of adult diapause will be examined to determine whether there is a causal relationship between the hormonal milieu of the diapausing mosquito and its patterns of sexual and host-seeking behavior.